This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Modern society is greatly dependent on the use of hydrocarbons for fuels and chemical feedstocks. Hydrocarbons are generally found in subsurface rock formations that can be termed “reservoirs.” Removing hydrocarbons from the reservoirs depends on numerous physical properties of the rock formations, such as the permeability of the rock containing the hydrocarbons, the ability of the hydrocarbons to flow through the rock formations, and the proportion of hydrocarbons present, among others.
Easily harvested sources of hydrocarbon are dwindling, leaving less accessible sources to satisfy future energy needs. However, as the costs of hydrocarbons increase, these less accessible sources become more economically attractive. For example, the harvesting of oil sands to remove hydrocarbons has become more extensive as it has become more economical. The hydrocarbons harvested from these reservoirs may have relatively high viscosities, for example, ranging from 8 API, or lower, up to 20 API, or higher. Accordingly, the hydrocarbons may include heavy oils, bitumen, or other carbonaceous materials, collectively referred to herein as “heavy oil,” which are difficult to recover using standard techniques.
Several methods have been developed to remove hydrocarbons from oil sands. For example, strip or surface mining may be performed to access the oil sands, which can then be treated with hot water or steam to extract the oil. However, deeper formations may not be accessible using a strip mining approach. For these formations, a well can be drilled to the reservoir and steam, hot air, solvents, or combinations thereof, can be injected to release the hydrocarbons. The released hydrocarbons may then be collected by the injection well or by other wells and brought to the surface.
A number of techniques have been developed for harvesting heavy oil from subsurface formations using thermal recovery techniques. Thermal recovery operations are used around the world to recover liquid hydrocarbons from both sandstone and carbonate reservoirs. These operations include a suite of steam based in situ thermal recovery techniques, such as cyclic steam stimulation (CSS), steam flooding, and steam assisted gravity drainage (SAGD).
For example, CSS techniques includes a number of enhanced recovery methods for harvesting heavy oil from formations that use steam heat to lower the viscosity of the heavy oil. The steam is injected into the reservoir through a well and raises the temperature of the heavy oil during a heat soak phase, lowering the viscosity of the heavy oil. The same well may then be used to produce heavy oil from the formation. Solvents may be used in combination with steam in CSS processes, such as in mixtures with the steam or in alternate injections between steam injections. These techniques are described in U.S. Pat. No. 4,280,559 to Best, U.S. Pat. No. 4,519,454 to McMillen, and U.S. Pat. No. 4,697,642 to Vogel, among others.
Another group of techniques is based on a continuous injection of steam through a first well to lower the viscosity of heavy oils and a continuous production of the heavy oil from a lower-lying second well. Such techniques may be termed “steam assisted gravity drainage” or SAGD. Various embodiments of the SAGD process are described in Canadian Patent No. 1,304,287 to Butler and its corresponding U.S. Pat. No. 4,344,485.
In SAGD, two horizontal wells are completed into the reservoir. The two wells are first drilled vertically to different depths within the reservoir. Thereafter, using directional drilling technology, the two wells are extended in the horizontal direction that result in two horizontal wells, vertically spaced from, but otherwise vertically aligned with the other. Ideally, the production well is located above the base of the reservoir but as close as practical to the bottom of the reservoir, and the injection well is located vertically 10 to 30 feet (3 to 10 meters) above the horizontal well used for production.
The upper horizontal well is utilized as an injection well and is supplied with steam from the surface. The steam rises from the injection well, permeating the reservoir to form a vapor chamber that grows over time towards the top of the reservoir, thereby increasing the temperature within the reservoir. The steam, and its condensate, raise the temperature of the reservoir and consequently reduce the viscosity of the heavy oil in the reservoir. The heavy oil and condensed steam will then drain downward through the reservoir under the action of gravity and may flow into the lower production well, whereby these liquids can be pumped to the surface. At the surface of the well, the condensed steam and heavy oil are separated, and the heavy oil may be diluted with appropriate light hydrocarbons for transport by pipeline.
The efficacy of harvesting of hydrocarbons from the oil sand reservoir depends on the maintenance of temperature to mobilize the hydrocarbons. The loss of steam pressure, for example, through permeable, fractured, or eroded sections of an overlying rock segment, or cap rock, will lower the pressure, lowering the temperature. Further, the lost steam may reach the surface, creating additional regulatory issues. Conversely, the influx of water through permeable, fractures or eroded sections would cause the steam chamber to collapse, effectively ending the SAGD process.
Various techniques have been used to form barriers to fluid flow, for example, in areas with missing cap rock formations. For example, US Patent Application Publication No. 2011/0186295, by Kaminsky, discloses a method for recovering viscous oil such as bitumen from a subsurface formation. An artificial barrier that is largely impermeable to fluid flow is created in a subterranean zone above or proximate to a top of the subsurface formation. The viscosity of the viscous oil and mobilizing hydrocarbons is reduced to form a readily flowable heavy oil by addition of heat and/or solvent. Heating preferably uses a plurality of heat injection wells. The heavy oil is produced using a production method that preserves the integrity of the artificial barrier.
As another example, in an application for a statutory consent test run to the Energy Resources Conservation Board (ERCB) of Canada, Cenovus requested permission to test a dewatering application to form an air barrier to isolate an aquifer overlying a hydrocarbon field. See ERCB, application Ser. No. 1,689,991 (Jun. 9, 2011). The test will use four lower horizontal wells arranged as a square to produce water from a region of the aquifer. Another horizontal well, located above and central to the four wells, is used to inject air to replace the water. The produced water is reinjected in four horizontal wells, above and offset to the exterior of the four horizontal production wells, and offset from the air injection well. The Cenovus technique may isolate the hydrocarbon region from an offset interval of permeable cap rock, but may allow the loss of pressure from, or the influx of water into, the field under certain conditions, such as during a power loss.
Canadian Patent Application Publication No. 2,463,110 describes a method for inhibiting migration of fluids into and/or out of a treatment area undergoing an in situ conversion process. Barriers in the formation proximate a treatment area may be used to inhibit the migration of fluids. Inhibition of the migration of fluids may occur before, during, or after an in situ treatment process. For example, migration of fluids may be inhibited while heat is provided from heaters to at least a portion of the treatment area. Barriers may include naturally occurring portions (e.g., overburden, and/or underburden) and/or installed portions, such as frozen barrier zones, cooled by a refrigerant.
The references cited above, among others, describe the use of freeze walls proximate to hydrocarbon reservoirs to isolate hydrocarbons from other portions of the subsurface. However, none of the references describes the use of freeze walls to form a chamber above the hydrocarbon reservoir. Such a chamber may provide a mechanism for trapping a gas cap over the reservoir.